Critical to the development of atherosclerotic lesions is the interaction between blood and vascular cells with components of the extracellular matrix (ECM). This proposal focuses on the role of an ECM protein, thrombospondin-4 (TSP-4). Our published studies and preliminary data strongly implicate TSP-4 in regulation of inflammation in the vessel wall, and, as a consequence, atherosclerosis is markedly suppressed in the TSP- 4 KO mouse. Mechanistically reduced expression of multiple leukocyte adhesion molecules and monocyte chemotactic protein (MCP-1) by endothelial cells (EC) in TSP-4 KO mice leads to few macrophages (M) accumulating into developing lesions, thereby suppressing a key event in atherogenesis. Furthermore, another EC response important in atherogenesis, angiogenesis, is suppressed in TSP-4 KO mice, the first evidence that TSP-4 is a pro-angiogeneic. Superimposed on these novel observations is our finding (now replicated in numerous independent studies) that a high frequency genetic variant, P387 TSP-4 as contrasted to A387, is an atherothrombotic risk factor. The primary hypothesis to be tested is that TSP-4 activates specific molecular mechanisms and pathways in vascular cells that regulate cell-matrix dynamics and vascular inflammation and that the P387 variant accentuates these pro-atherogenetic responses, including angiogenesis. A new knock-in mouse expressing P387 TSP-4 variant will permit testing this hypothesis in vivo. Three specific aims are proposed: 1) To define the role of the TSP-4 variants in atherosclerosis using TSP-4 KO and P387 TSP-4 K-In mice and to perform bone marrow transplantation in combination with in situ hybridization to determine if differences in atherosclerosis are dependent on blood and/or vascular cells. 2) To identify the molecular mechanisms underlying the differential responses of EC to the TSP-4 variants and characterize the proangiogenic activity of TSP-4 in vivo using TSP-4 KO and P387 TSP-4 K-In mice. 3). To perform translational studies to determine relationships between TSP-4, atherogenesis and angiogenesis in lesioned and non-lesioned areas of human coronary arteries. We found a new plasma biomarker, TSP-4RA, that was markedly elevated in a small panel of AMI patients. This lead will be followed to determine if TSP-4RA is selectively elevated in AMI patients, is prognostic for second AMI, and, at levels attained in patients, influences cellular responses. Our overall goals are to establish the roles of TSP-4 in vascular cell biology, to identify the molecular mechanisms underlying its proatherogenic and proangiogenic functions, and to determine whether these functions are enhanced by the P387 TSP-4 variant in mouse and human studies.